Generally, exhaust gas emitted by internal combustion engines contains particulate matter (PM) having carbon as a main component, as well as ash made up of unburned components, which are known to give rise to air pollution. Regulations concerning emissions of particulate matter have therefore become stricter year after year, alongside regulations on harmful components in exhaust gas such as hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), and the like. In this respect, various technologies for trapping and removing such particulate matter from exhaust gas have been proposed.
For instance, particulate filters for trapping such particulate matter are provided in the exhaust passage of internal combustion engines. In gasoline engines, for example, a certain amount of particulate matter, though smaller than that in diesel engines, is emitted together with exhaust gas, and a gasoline particulate filter (GPF) may in some cases be fitted in the exhaust passage. Such particulate filters include known filters having a structure, referred to as a wall flow type, in which a substrate is configured of multiple cells made up of a porous substance, and in which the inlets and the outlets of the multiple cells are plugged alternately (Patent Literature 1 and 2). In a wall flow-type particulate filter, exhaust gas that flows in through cell inlets passes through partitioning porous cell partition walls, and is discharged out through the cell outlets. As the exhaust gas passes through a porous cell partition wall, the particulate matter is trapped within the pores inside the partition wall.